The favorable working point for a solar cell is the maximum power point MPP at which the solar cell delivers the maximum output power. FIG. 1 shows the typical current-voltage characteristics (I-V characteristics) of a solar cell according to the prior art. The output power of the solar cell it is given by the product of I and V and its maximum defines the MPP.
A typical method for determining the MPP is to vary a test load that is coupled in series to the real load of the solar cell. The output power is calculated while the test load is varied. Further, the first order derivative of the product of I and V (i.e. the output power of the solar cell) is determined. As might be seen from the output power curve in FIG. 1, its first order derivative changes its slope at MPP. By increasing the current I, when starting at low current values, the corresponding voltage V increases until the output power reaches its maximum. A further increase of the voltage V leads to decreasing values of the current I. In other words, depending on whether the respective I and V values are located left or right from the MPP, the first order derivative has a positive or a negative slope. While a positive slope involves the information that the MPP will be found at higher current values, a negative slope indicates that the MPP is located at lower current values. By adjusting the current and voltage accordingly, the solar cell can be set to its MPP.
However, a plurality of external parameters influences the output power characteristics of a solar cell. Especially, its exposure to light and the working temperature of the cell change the output power curve. Consequently, also the MPP is shifted to different voltage and current values. This dependencies lead to the necessity to adjust the working current and voltage of the solar cell frequently, in order to always operate the solar cell at its MPP. This adjustment is usually performed by a microcontroller unit or a digital signal processor that operates in the aforementioned way. The adjustment must be fast since not only the real load of the solar cell can vary quickly, but also the operating conditions like light exposure and working temperature may be varying. However, the application of microcontroller units or digital signal processors raises the system costs since these components are expensive.